The present invention relates to a method for testing the integrity of filter elements.
Filtration systems, particularly those for sterile filtration, are tested for their integrity prior to and after filtration to ensure filtration reliability and product safety. This includes a diffusion test, a pressure retention test and a bubble point test, which are also approved for use in pharmaceutical applications, with special safety requirements regarding sterile filtration. These tests correlate to destructive load tests, the so-called bacteria challenge tests, in which standard test bacteria are used to determine the degree of sterile filtration by standardized test methods (ASTM 838-83).
The diffusion test measures the transmembrane diffusive flow of a gas (Fick's first law) through a wetted filter material, e.g., a membrane, established after the filter material is pressurized on one side with a test gas under a test pressure. To conduct the test, a filter element e.g., a membrane filter cartridge, which is wetted with a liquid, e.g., water, alcohol or a product fluid, is pressurized with a test gas under a test pressure, typically air or nitrogen. Various methods are used to measure the gas diffusion, e.g., measuring the gas volume on the clean side (filtrate side) or the amount of gas that needs to be delivered on the inflow side (retentate side) to maintain the test pressure, or determining the pressure increase on the clean side or the pressure drop on the inflow side after switching off the pressure gas source (pressure retention test).
In small-area systems the volumetric gas flow rate caused by genuine defects in the filter materials can be clearly detected based on its order of magnitude. This is no longer possible, however, in large area systems with many filter elements connected in parallel or with large filter cartridges that currently have filter areas of up to 90 m2. This is due to the fact that, when the test is conducted, no distinction can be made between the gas flow component produced by diffusion and the component based on convection caused by minor but harmful defects. Particularly in large filter areas, the diffusion component thus frequently overlaps the convection component, so that the measured total gas flow makes it appear as if the filter elements were intact.
Jornitz, published German Patent Application No. DE 199 18 419 describes a method for testing the integrity of filter elements using a combination of the diffusion test and the bubble point test. With this combination, a statement is to be made regarding the integrity of the filter elements, if the gas flow caused by diffusion does not exceed the maximum allowable value of the gas flow at the corresponding test pressure.
Weich, U.S. Pat. No. 5,417,101 (=DE 41 19 040) discloses a method and apparatus for testing the operational state of filter elements. The filter elements are present in a filter system that has a plurality of parallel filter elements, which in turn are divided into a plurality of blockable sections. According to the method proposed by U.S. Pat No. 5,417,101, the full gas flow rate of a test gas under a test pressure through the wetted filter material is measured collectively for all filter elements. Thereafter it is determined whether the measured flow rate differs from a first desired flow rate by an amount that falls within a first predefined range. The desired flow rate corresponds to the situation where the filters are intact. Any deviation that falls within the predefined range is said to indicate that all filter elements are intact. This method has the drawback that the desired inference is not unambiguous since, in such a case, defective filter elements may be present in the filter housing but their unacceptable increased gas flow cannot be detected by the collective measurement of the total flow rate.
If, however, the measured total flow rate exceeds the first desired flow rate by an amount that is greater than the amount allowed by the first predefined range, this should mean that at least one filter element is not intact. In this case, the passage of the gas through at least one segment is blocked and the total gas flow rate through the wetted filter material of the remaining filter elements is measured. Now it is determined whether this measured flow rate differs from a second desired flow rate that corresponds to the resulting reduced number of filter elements by an amount that falls within a second predefined range. A deviation in the second predefined range should indicate that one or more filter elements in the blocked segment is/are not intact.
This procedure again has the drawback that this inference, too, is not unambiguous, since in such a case defective filter elements may be present in the non-blocked segments but their unacceptably increased gas flow rate cannot be detected by the collective measurement of the flow rate. Thus, the method proposed by U.S. Pat. No. 5,417,101 does not produce unambiguous results with respect to the integrity of the filter elements.
In summary it may therefore be said, especially when filter systems with a plurality or many filter elements are tested using the methods of the prior art, that in case of doubt, each filter element must be individually tested for its integrity.